ssize_t rio_readnb(rio_t *rp,void *usrbuf,size_t n)
{
    size_t nleft = n;
    sszie_t nread;
    char *bufp = usrbuf;
    while(nleft >0)
    {
        if ((nread = rio_read(rp,bufp,bleft))<0)
        {
            if (errno == EINTR)
                nread =0;
            else
                return -1;
        }
        else if (nread == 0)
            break;
        nleft -= nread;
        bufp += nread;
    }
    return (n-nleft);
}
